1. Field of the Invention
The present invention relates to a white dot out detecting device incorporated into an optical disk apparatus or the like and serving to detect a white dot out (or white drop out). The white dot out is a phenomenon in which a volume of reflected light is amplified and a potential on a bright side thereby temporarily soars due to a defect on the optical disk (a part on the optical disk where write and read operations cannot be normally executed).
2. Description of the Related Art
In recent years, an information volume handled in a computer system has been significantly increasing, in response to which an optical disk apparatus capable of executing a high-rate operation and random accesses and having a large capacity is widely used as an information data recording/reproducing apparatus. In the optical disk apparatus, optical disks such as generally-called CD-R, CD-RW, DVD-R/RW, DVD-RAM and BLU-RAY are used as a recording medium.
Some optical disk apparatuses are provided with a white dot out detecting device. In the white dot out detecting device, in general, an optical beam is convergently irradiated on the optical disk and an envelope variation of an information signal in accordance with an intensity of a reflected light from the optical disk is detected so that the white dot out on the optical disk is detected and a white dot out detection signal showing the presence/absence of the white dot out is outputted. The white out dot detection signal is utilized by a servo circuit for controlling a tracking servo and a focus servo with respect to the optical disk as a signal for holding a previous value. The white dot out detection signal is also utilized for obtaining an extracted signal for judging a non-recordable region on the optical disk by means of a CPU incorporated into the optical disk apparatus in order to execute various controls.
FIG. 11 is a block diagram illustrating a constitution of a conventional white dot out detecting device, an example of which is recited in No. 2003-196853 of the Publication of the Unexamined Japanese Patent Applications. In FIG. 11, a reference symbol WG denotes a recording/reproduction control signal showing that the optical disk is currently subjected to a recording process or a reproducing process. A reference numeral 11 denotes a variable gain amplifier for amplifying an information signal AS in accordance with the intensity of the reflected light from the optical disk by means of a gain in accordance with the recording/reproduction control signal WG into a predetermined amplitude. A reference numeral 12 denotes a high-rate envelope detecting circuit for detecting an envelope of an amplitude output signal AP from the variable gain amplifier 11. A reference numeral 13 denotes a first limit circuit for controlling a voltage of an envelope signal EM from the high-rate envelope detecting circuit 12 to stay within a predetermined limit voltage. A reference numeral 14 denotes an integrating circuit comprising a resistance R1 and a capacitance C1, wherein the envelope signal EM outputted from the high-rate envelope detecting circuit 12 is integrated. A reference numeral 15 denotes a slice level setting circuit for generating a slice level SD based on an integration signal IS from the integrating circuit 14. A reference numeral 16 dentoes a second limit circuit for controlling a voltage of the slice level SD from the slice level setting circuit 15 to stay within a predetermined limit voltage. A reference numeral 17 denotes a comparator for comparing a first limit output signal LM1 from the first limit circuit 13 and a second limit output signal LM2 from the second limit circuit 16 to each other and activating and outputting a white dot out detection signal DD when the first is larger than the latter.
Next, an operation of the conventional white dot out detecting device is described referring to a waveform chart of FIG. 12. The optical beam is convergently irradiated on the optical disk and the information signal AS in accordance with the intensity of the reflected light from the optical disk is inputted to the variable gain amplifier 11. The variable gain amplifier 11 amplifies the information signal AS into the predetermined amplitude by means of the gain in accordance with the recording/reproduction control signal WG. The variable gain amplifier 11 outputs the amplified information signal AS to the high-rate envelope detecting circuit 12 as the amplitude output signal AP.
A level of the reflected light from the optical disk differs in the recording and reproducing operations. The variable gain amplifier 11 diminishes the difference in the levels so that it is not detected as an envelope variation. The variable gain amplifier 11 uses a level of the volume of the reflected light on a bright side as the limit voltage.
The high-rate envelope detecting circuit 12 detects the envelope of the inputted amplitude output signal AP and outputs the envelope signal EM thereby obtained to the first limit circuit 13 and the integrating circuit 14. When the white dot out is generated, the envelope signal EM includes a white dot out (WDO) component.
The first limit circuit 13 controls the limit voltage when the white dot out component included in the envelope signal EM reaches a level way beyond the bright-side level. Thereby, the white dot out component is controlled so as to stay within an input dynamic range of the comparator 17.
The integrating circuit 14 integrates the envelope signal EM from the high-rate envelope detecting circuit 12 and outputs the integration signal IS to the slice level setting circuit 15. The slice level setting circuit 15 level-converts the integration signal IS to thereby generate the slice level SD.
The second limit circuit 16 controls the limit voltage when the slice level SD reaches a level far beyond the bright-side level. Thereby, the white dot out component is controlled so as to stay within the input dynamic range of the comparator 17.
The comparator 17 binarizes the first limit output signal IM1 from the first limit circuit 13 based on the second limit output signal LM2 from the second limit circuit 16 as a reference to thereby generate and output the white dot out detection signal DD.
It is assumed here that an envelope of the information signal AS is undergoing a drastic change due to the presence of the white dot out (WDO) on the optical disk. Such a state is, for example, shown in a period T2 in FIG. 12. Because a time constant of the high-rate envelope detecting circuit 12 in the foregoing state is small, the first limit output signal LM1 resulting from the envelope signal EM inputted to the first limit circuit 13 consequently shows a waveform in response to the drastic change of the envelope. The integration signal IS resulting from the envelope signal EM inputted to the integration circuit 14 does not follow the drastic change of the information signal AS and shows a waveform subjected to a slow change (see P1). Therefore, the second limit output signal LM2 from the second limit circuit 16 shows the slowly-changing waveform in the same manner. As a result, the effective white dot out detection signal DD, such as TS1, is outputted from the comparator 17 (see P 2-3).
However, it was not possible to completely eliminate the difference in the levels of the reflected light in the recording and reproducing operations in the conventional white dot out detecting device due to such a reason that the gain setting in the variable gain amplifier 11 includes some variability. Because of the inadequacy, the level difference is generated in the amplitude output signal AP of the variable gain amplifier 11 when the operation with respect to the optical disk shifts from the recording to the reproduction or from the reproduction to the recording.
In particular, when the operation with respect to the optical disk shifts from the reproduction to the recording, the envelope signal EM promptly follows the change of the information signal AS, while the integration signal IS requires sometime to follow the change (see P31). Therefore, the second limit output signal LM2 is lower than the first limit output signal LM1 for a lengthened period of time, and a false white dot out detection signal DD, such as FS1, is outputted for a long period of time (see P32→33). Therefore, the reproduction cannot be carried out in a stable manner. Further, when the operation with respect to the optical disk is shifted from the recording to the reproduction, though the false white dot out detection signal DD is not outputted, the detection of the white dot out cannot be soon restarted immediately after the shift of the recording/reproduction. As a result, the stability in the detection of the white dot out is deteriorated (see P34 and a TF3 period).